Then came his enunciation of the grand principle of economy in the construction and operation of the steam engine: "Keep the cylinder as hot as the steam which enters it," as he expressed it. This was Watt's guiding principle, as it has been that of all his successors in the improvement of the economic performance of the steam engine and of all other heat engines. The great source of waste is the dispersion of heat, uselessly, which should be applied to the production of work by its transformation, thermodynamically, into the latter form of energy. The second form of waste is that of power thus produced in the unprofitable work of moving the parts of the engine itself; and the third is that of heat by transfer, without transformation, by conduction and radiation to surrounding bodies. In modern engines, the latter is but three or five per cent., in the best cases; the second waste constitutes perhaps ten per cent.; while the first of these losses amounts very usually to seventy per cent., of which last one-third or one-fourth is of the kind discovered by Watt, the rest being the thermodynamic waste incident to all known methods of operation of heat engines, and apparently unavoidable. In our very best and largest engines, the waste found by Watt to constitute three fourths of all heat supplied has been brought down to ten per cent., a fact which well exemplifies the advances made since his time of apprenticeship by himself and his successors of this nineteenth century. The steam engine of to-day, in its most successful operation, gives us twenty-five times as much power from a pound of coal as did the engine that the great inventor sought to improve: this is the magnificent fruit of that one discovery of James Watt, and of application of the simple principle which he so concisely and clearly stated.